The present invention relates generally to the inspection of semiconductor wafers and more particularly, to a method and apparatus for inspecting a surface of a semiconductor wafer having repetitive patterns for contaminant particles.
Integrated circuits (ICs) are commonly manufactured through a series of processing steps. Very often more than a hundred processing steps are performed to produce a properly functioning integrated circuit chip.
A semiconductor material, commonly in the shape of a wafer, serves as the substrate for integrated circuits. Semiconductor ICs are typically manufactured as an assembly of a hundred or more chips on a single semiconductor wafer, which is then cut up to produce the individual IC chips. Typically, a wafer made of silicon is used as the integrated circuit substrate, the silicon wafer being approximately 150-200 mm in diameter and 0.5-1 mm thick. During the manufacturing process, the silicon wafer is first polished and cleaned to remove any contaminant particles situated thereon. The silicon wafer is then is treated in preparation for a series of processing steps involving a plurality of photolithographic patterns (also commonly referred to as masks). In the production of integrated circuits, microelectronic circuits are formed onto the silicon wafer through a process of layering. In the layering process, conductive and insulative layers of thin films are deposited and patterned onto the silicon wafer. Each layer is patterned by a mask designed specifically for it, the mask defining the areas within the wafer that are to be treated such as by etching or implanting.
Semiconductor fabrication technology today deals with silicon wafers which are approximately 200 mm in diameter and which feature geometries with dimensions well below 1 xcexcm (micrometer). Due to the high complexity and level of integration of integrated circuits, the absence of contaminants on every layer of the wafer is critical in order to realize acceptable levels of product yield. However, it has been found that contaminant particles are often introduced onto the semiconductor wafer during the manufacturing process of integrated circuits. As a consequence, the presence of one contaminant particle larger than the half the width of a conductive line on the silicon wafer can result in complete failure of a semiconductor chip produced from the wafer. Such a wafer has to be discarded which thereby decreases the percentage yield per wafer and increases the overall cost of the individual chips. Therefore, a critical task facing semiconductor process engineers is to identify and, as far as possible, to eliminate sources of surface contamination on each layer of the semiconductor wafer.
Accordingly, inspection systems are well known in the art and are commonly used to detect, view, identify and correct yield limiting defects introduced in the fabrication process of integrated circuits. Wafer inspection systems often include a light source, such as a laser, and a light sensitive imaging camera, or detector. In use, the light source is used to scan the surface of the wafer by means of illuminating particular regions of the surface of the wafer. The light sensitive imaging camera is positioned relative to the wafer to pick up scattered light for display on a viewing screen for further analysis. The imaging camera creates a visual for the viewing screen based on the number of photons which disperse from the wafer as the laser performs its scanning function. The visual could equivalently be formed by use of a non-imaging detector (e.g. a photomultiplier tube) with appropriate means of scanning for image formation. The camera will detect light scattered from any contaminant particles situated on the wafer, the intensity of the scattered light being generally proportional to the size of the particles, wherein the larger particles generally reflect more photons onto the imaging camera than smaller particles. As a consequence, larger particles will produce a brighter image and will have a greater light intensity than smaller particles.
Inspection systems of the type described above have been made commercially available by such companies as Inspex, Inc. of Billerica, Mass.
In U.S. Pat. No. 4,772,126 to C. D. Allemand et al, there is disclosed an apparatus and method for detecting the presence of particles on the surface of an object such the front side of a patterned semiconductor wafer. A vertically expanded, horizontally scanning, beam of light is directed onto an area on the surface of the object at a grazing angle of incidence. A video camera positioned above the surface detects light scattered from any particles which may be present on the surface, but not specularly reflected light. The surface is angularly prepositioned (rotated) relative to the incident light beam so that the diffracted light from the surface and the pattern of lines on the surface is at a minimum. The object is then moved translationally to expose another area to the incident light beam so that the entire surface of the object or selected portions thereof can be examined, an area at a time.
In U.S. Pat. No. 5,659,390 to J. J. Danko, there is disclosed a method and apparatus for detecting particles on a surface of a semiconductor wafer having repetitive patterns. The apparatus for detecting particles on the front surface of a patterned semiconductor wafer having repetitive patterns includes a laser for illuminating an area on the front surface at grazing angle of incidence with a beam of polarized light. A lens collects light scattered from the area and forms a Fourier diffraction pattern of the area illuminated. A Fourier mask blocks out light collected by the lens at locations in the Fourier diffraction pattern where the intensity is above a predetermined level indicative of background information and leaves in light at locations where the intensity is below the threshold level indicative of possible particle information. The Fourier mask includes an optically addressable spatial light modulator and a crossed polarizer with the Fourier diffraction pattern being used as both a read beam and a write beam for the spatial light modulator. A camera detects scattered light collected from the area by the lens and not blocked out by the Fourier mask.
Although widely used in commerce, inspection systems of the type described above have been found, on occasion, to be unsatisfactory in detecting the majority of notable particles disposed on the substrate. Rather, inspection systems of the type described above have been found, on occasion, to detect only a small fraction of the total number of notable defects on the substrate, which is highly undesirable.
The effectiveness in which an inspection system can detect particles on a substrate is dependent upon certain characteristics. As a first characteristic, the intensity of the detectable scattered light is dependent upon the physical and geometrical attributes of the defect, such as the size, shape, orientation and/or index of refraction of the particle. As a second characteristic, the intensity of the detectable scattered light is dependent upon the light intensity of the surrounding environment, or background, of the wafer. As a third characteristic, the intensity of the detectable scattered light is dependent upon the spatial relation of the light source and the detector relative to the substrate.
Accordingly, different approaches have been utilized to improve the effectiveness in which an inspection system can detect particles disposed on a substrate.
One approach which has been used to improve the effectiveness in which an inspection system can detect particles disposed on a substrate, thereby increasing the overall defect count of the system, is simply to analyze the detectable scattered light at different points of observation using multiple detectors. In this manner, with each observation point offering a different perspective on the scattered light, the ability of the inspection system to be able to distinguish the defect from the background of the wafer is enhanced.
The use of multiple detectors in an inspection system to analyze detectable scattered light at different points of observation introduces numerous drawbacks.
As a first drawback, it should be noted that no single multi-detector configuration will optimize the detection capability of the inspection system for all types of defects and substrates. Rather, one multi-detector configuration that is optimized for one substrate may lead to totally unacceptable results for another substrate, thereby rendering the inspection system unreliable.
As a second drawback, it should be noted that the capability to reconfigure the detectors on demand (e.g. after a change to a substrate with a distinctively different background scatter signature) is highly labor intensive, which is undesirable. Specifically, the optimization process for reconfiguring the detectors requires the implementation of a complicated search algorithm, the cumbersome repositioning of the detectors and the implementation of hardware for suppressing background light.
As a third drawback, it should be noted that the use of multiple detectors in an inspection system to analyze detectable scattered light at different points of observation is useless if the particle is not properly illuminated by the light source.
In U.S. Pat. No. 5,046,847 to T. Nakata et al, there is disclosed a method and apparatus for detecting foreign matter on a sample by illuminating a stripe-shaped region with linearly polarized light. Some of the light reflected by the sample is intercepted by a light intercepting stage, and the rest of the light reflected by the sample, which passes through the light intercepting stage is directed to a detecting optical system, to be detected by a photodetector. The sample is illuminated obliquely at a predetermined angle with respect to a group of straight lines constituting a primary pattern on the sample. The angle is selected so that the diffraction light reflected by the group of straight lines does not enter the detecting optical system. A polarizing spatial filter using a liquid crystal element may be disposed in a predetermined restricted region in a spacial frequency region, or Fourier transformation plane, within the detecting optical system. The light scattered by the sample may further be separated in the detecting optical system into partial beams having different wave orientation characteristics, which characteristics are detected by a number of one-dimensional solid state imaging elements. The signals are processed by a driver, adder, and quantizer in synchronism with the one-dimensional solid state imaging elements.
It should be noted that the inspection system disclosed in U.S. Pat. No. 5,046,847 to T. Nakata et al suffers from a notable drawback. Specifically, during the scanning process, the multiple light sources move dependently of one another and, as such, are orientated to have identical approach angles. The use of identical approach angles for each light source ensures that particles illuminated by each light source have common coordinates on the detector. However, it has been found that, by using identical approach angles for each light source, the effectiveness of each light source to illuminate particles is compromised. Specifically, the optimal approach angle for the first light source may be a less than optimal approach angle for the second light source. As a consequence, the benefit in using multiple light sources to increase the intensity of the illumination of particles on the wafer is not maximized, which undesirable.
In U.S. Pat. No. 4,966,457 to F. Hayano et al, there is disclosed a defect inspecting apparatus for determining the presence of a defect element adhering to either of the front and back surfaces of a thin film-like object to be inspected (the object having a light-transmitting property), wherein a single light beam made up of two light beams of different wavelengths is applied to a surface of the object and the incident angle of the combined light beam is varied. A first photoelectric detector receives light reflected by or transmitted by the object, and a second photoelectric detector receives light scattered by the defect element. A discriminator determines the surface of the object to which the defect element adheres based on detection outputs of the photoelectric detectors.
Other patents of interest include U.S. Pat. No. 4,895,446 to M. A. Maldari et al, U.S. Pat. No. 4,898,471 to J. P. Stonestram et al, U. S. Pat. No. 5,355,212 to K. B. Wells et al and U.S. Pat. No. 5,625,193 to S. V. Bronde et al.
It is an object of the present invention to provide a new and improved method and apparatus for inspecting a semiconductor wafer having repetitive patterns for contaminant particles.
It is another object of the present invention to provide a new and improved method and apparatus for inspecting a semiconductor wafer having repetitive patterns for contaminant particles by illuminating an area on the semiconductor wafer and detecting light scattered thereby.
It is yet another object of the present invention to provide a method and apparatus as described above which improves the overall detection count of contaminant particles on the semiconductor wafer.
It is still another object of the present invention to provide a method and apparatus as described above which effectively illuminates an area disposed on the semiconductor wafer for detection.
It is another object of the present invention to provide a method and apparatus as described above which has a limited number of parts and which is easy to use.
It is a further object of the present invention to provide a method and apparatus as described above for inspecting a semiconductor wafer having repetitive patterns for contaminant particles by illuminating an area on the semiconductor wafer, detecting light scattered from the surface and masking off light scattered from the pattern on the surface.
Accordingly, there is provided an apparatus for detecting the presence of contaminant particles on a surface of a semiconductor wafer having repetitive patterns, said apparatus comprising a light source adapted to produce a first beam of light and a second beam of light, said first beam of light being disposed to illuminate an area on the semiconductor wafer at a first approach angle which is angularly adjustable and a first angle of incidence which is angularly adjustable, said second beam of light being disposed to illuminate the same area on the semiconductor wafer at a second approach angle which is angularly adjustable and a second angle of incidence which is angularly adjustable, the first approach angle and the first angle of incidence of said first beam of light being adjustable independent of the second approach angle and the second angle of incidence of said second beam of light, respectively, an imaging detector disposed to detect light scattered from the area illuminated but not light specularly reflected from the area illuminated, an imaging lens for imaging the area illuminated on said imaging detector, said imaging lens having a Fourier plane, and a spatial filter disposed in the Fourier plane of said imaging lens for masking off the diffraction pattern produced by the background scatter from the surface of the semiconductor wafer.
There is also provided a method for detecting the presence of contaminant particles on a semiconductor wafer having repetitive patterns, said apparatus comprising illuminating an area on the semiconductor wafer with first and second beams of light, said first beam of light striking the semiconductor wafer at a first approach angle which is angularly adjustable and a first angle of incidence which is angularly adjustable, said second beam of light striking the semiconductor wafer at a second approach angle which is angularly adjustable and a second angle of incidence which is angularly adjustable, said first approach angle and said first angle of incidence being adjustable independent of said second approach angle and said second angle of incidence, respectively, setting said first and second approach angles to maximize the detection of defects of interest, positioning an imaging detector above the semiconductor wafer for detecting at least some of the light scattered from the area illuminated but not specularly reflected light, providing an imaging lens for imaging said area illuminated on said imaging detector, providing a spatial filter in the Fourier plane of the imaging lens for masking off the diffraction pattern produced by the background on the semiconductor wafer from a first one of the two beams of light, and adjusting said angle of incidence of the other beam of light so that the diffraction pattern formed by the other beam of light in the Fourier plane overlaps the diffraction pattern formed by the first beam of light. As a result, a spatial filter constructed to block off the background diffraction pattern from the first beam also blocks off the diffraction pattern formed by the background from the second beam.
Various other features and advantages will appear from the description to follow. In the description, reference is made to the accompanying drawings which form a part thereof, and in which is shown by way of illustration, a specific embodiment for practicing the invention. The embodiment will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. The following detailed description is therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims.